Let's Encrypt certificate used in malversiting

Posted by   Virus Bulletin on   Jan 8, 2016

We'd better get used to a world where malicious traffic is encrypted too.

According to some people, myself included, Let's Encrypt was one of the best things that happened to the Internet in 2015. Now that, as of December, the service is in public beta, anyone can register certificates for domains they own, in a process that is both easy and free.

Cybercriminals have noticed this too, and rather unsurprisingly, Trend Micro reports that a certificate issued by Let's Encrypt was used in an Angler exploit kit-powered malvertising campaign, to make the malicious advertisements harder to detect.

  The malvertising taking place. Source: Trend Micro.

What makes this case particularly interesting is the fact that the domain for which the certificate was issued was the subdomain of a legitimate site, whose DNS was compromised. As domain-based reputation isn't usually granular enough to distinguish between subdomains, this could have helped them avoid detection even further.

Let's Encrypt only issues Domain Validation certificates, which don't do more than validate the domain; hence it doesn't believe it needs to police the content of the domains for which it issues certificates, although as a possibly temporary compromise, the domains are checked against Google's Safe Browsing API before a certificate is issued.

I agree with Let's Encrypt here. I think our goal should be to encrypt all Internet traffic, and if bad traffic gets encrypted too, then that is a feature of the system, not a bug. Given how easy it is to register certificates, more policing would simply lead to a cat-and-mouse game. And there is also the danger of a slippery slope, where governments and interest groups start to pressure Let's Encrypt to revoke the certificates of sites they perceive as bad.

We'll just have to accept that more and more traffic is encrypted and find ways to block malicious activity in an environment where all traffic is encrypted.

Of course, this particular case is a little different: the exploit kit users in this case didn't "own" the subdomain; they were merely able to point it to their own server. It might be worth Let's Encrypt considering an automatic way in which domain owners can revoke certificates issued to subdomains. But that may well complicate the whole process and make little impact in practice. After all, for a successful malware campaign, domains only need to be active for a very short period of time.

If you've been telling people that the mere presence of a 'lock icon' in the address bar is a sign that a site is harmless, now is really the time to stop doing that.



Posted on 08 January 2016 by Martijn Grooten
twitter.png
fb.png
linkedin.png
hackernews.png
reddit.png

 

Latest posts:

New article: Run your malicious VBA macros anywhere!

Kurt Natvig explains how he recompiled malicious VBA macro code to valid harmless Python 3.x code.

New article: Dissecting the design and vulnerabilities in AZORult C&C panels

In a new article, Aditya K Sood looks at the command-and-control (C&C) design of the AZORult malware, discussing his team's findings related to the C&C design and some security issues they identified.

VB2021 localhost call for papers: a great opportunity

VB2021 localhost presents an exciting opportunity to share your research with an even wider cross section of the IT security community around the world than usual, without having to take time out of your work schedule (or budget) to travel.

New article: Excel Formula/Macro in .xlsb?

In a follow-up to an article published last week, Kurt Natvig takes us through the analysis of a new malicious sample using the .xlsb file format.

New article: Decompiling Excel Formula (XF) 4.0 malware

In a new article, researcher Kurt Natvig takes a close look at XF 4.0 malware.

We have placed cookies on your device in order to improve the functionality of this site, as outlined in our cookies policy. However, you may delete and block all cookies from this site and your use of the site will be unaffected. By continuing to browse this site, you are agreeing to Virus Bulletin's use of data as outlined in our privacy policy.